1. Field of the Invention
The present invention relates to a DC motor with brushes. More particularly, the present invention relates to the structure of brushes of a DC motor with brushes.
2. Description of Related Art
A typical DC motor with brushes has a brush holder 51 made of synthetic resin and a pair of brushes 55 and 56 retained by the brush holder 51, as shown in FIG. 9(A). Terminals 571 and 572 are connected to base sections of the respective pair of brushes 55 and 56. Required wirings are provided on the terminals 571 and 572. The pair of brushes 55 and 56 extend in directions that define an angle of 90xc2x0 and are brought in flexible contact with an outer peripheral surface of a commutator 52. Throughout the present specification, in the case of the structure described above, the brush 55 that is located outside is referred to as an outside brush, and the brush 56 having an end section extending toward an intermediate section of the outside brush in a longitudinal direction is referred to as an inside brush.
The inside brush 56 is slightly shorter than the outside brush 55 so as not to interfere with the outside brush 55. In the prior art shown in this figure, the outside brush 55 is used for a positive pole and the inside brush 56 is used for a negative pole.
In the small sized DC motor with brushes thus structured, directions of the brushes 55 and 56 to be disposed and surfaces of the brushes to which the terminals 571 and 572 are attached are determined by several factors including, for example, rotational directions of the motor (the commutator 52). For example, in the motor shown in FIG. 9(A), the motor rotates in a counterclockwise direction as indicated by an arrow CCW. When the motor rotates in a counterclockwise direction, the brushes 55 and 56 are disposed in a manner that the commutator 52 rotates in directions toward the tips of the end sections of the brushes 55 and 56. Also, the terminals 571 and 572 are affixed to the brushes 55 and 56 on their surfaces that contact the commutator 52.
As shown in FIG. 9(B), when the motor rotates in a clockwise direction as indicated by an arrow CW, the positions of the brushes 55 and 56 are changed in a manner that the commutator 52 also rotates in a direction toward the tips of the end sections of the brushes 55 and 56. The terminals 571 and 572 are affixed to the opposite surfaces of the brushes 55 and 56 shown in FIG. 9(A).
In the brush structure used in the conventional DC motor with brushes, the inside brush 56 is designed to be shorter so that the outside brush 55 and the inside brush 56 do not interfere with each other. As a result, the brushes 55 and 56 for a positive pole and a negative pole cannot be formed from a common part. In fact, the brushes 55 and 56 should preferably be formed in the same size. There are delicate relations between the length of the brushes and contact points between the brushes and the commutator. Portions of the brushes that extend beyond the contact points on the commutator 22 are required to be in a specified ratio with respect to the entire length of the brushes. However, if the brushes are formed in the same size, the tip of the inside brush 56 contacts the outside brush 55 and thereby interference between the two brushes takes place.
If the inside brush 56 is made shorter, the tip portion of the inside brush 56 becomes too light. As a result, when the motor rotates at high speed, the tip section of the inside brush 56 tends to jump from the peripheral surface of the commutator 22, in other words, the tip section of the inside brush 56 tends to be separated from the peripheral surface of the commutator 22.
It is an object of the present invention to provide a DC motor with brushes in which positions of brushes on a brush holder are improved such that brushes for positive and negative poles can be formed from a common part.
In accordance with one embodiment of the present invention, a DC motor with brushes may include a rotary shaft, an armature rotatable with the rotary shaft, a stator magnet disposed opposite to the armature, a commutator affixed on the side of the armature, and a pair of brushes retained by a brush holder. The brushes are retained by the brush holder in a manner that the brushes are brought in contact with a peripheral surface of the commutator. In one aspect of the embodiment of the present invention, the pair of brushes has an identical shape. The pair of brushes is composed of an outside brush and an inside brush. The outside brush is bend in a direction in which the outside brush separates from the other brush when the pair of brushes is retained by the brush holder, and is brought in contact with the commutator. The inside brush has an end section extending toward an intermediate point of the outer brush in a longitudinal direction thereof. Each of the outside brush and the inside brush has a base section that is affixed to a terminal and an end section that is in contact with the commutator wherein the end section is bent with respect to the base section such that each of the brushes generally assumes a L-shape.
In accordance with the embodiment of the present invention, the pair of brushes does not interfere with one another. Therefore, one of the brushes does not have to be made shorter than the other brush to avoid interference between the two brushes. Accordingly, a pair of brushes having the same shape can be used, thus eliminating the time-consuming work for adjusting the brushes. Since positive and negative poles can be formed from common brushes, the parts management is readily performed.
Also, each of the outside brush and the inside brush has a base section to which a terminal is attached and an end section that is brought in contact with the commutator wherein the end section is bent with respect to the base section so that each of the brushes generally defines an L-shape. The use of the L-shaped brush allows an effective placement of the brushes within a small space.
In accordance with one embodiment of the present invention, the pair of brushes may be formed from an outside brush and an inside brush having an identical shape. An end section of the inside brush extends in a direction pointing toward an intermediate section of the outside brush in a longitudinal direction. When such inside brush and outside brush are used, the brush holder may preferably have a protrusion that comes in contact with the outside brush and outwardly bends the outside brush so that the outside brush does not contact the inside brush when the outside brush is mounted on the brush holder and before the commutator is attached to the brush holder. With this structure, the outside brush outwardly bends when the outside brush is attached to the brush holder. As a result, even when the inside brush and the outside brush have the same size, the brushes do not interfere with each other before the commutator is disposed. Although the inside brush extends toward the outside brush, common brushes can be used as a pair. Also, the inside brush does not have to be made shorter, and therefore the end section of the brush can be made heavier. As a consequence, the brush continues to contact the peripheral surface of the commutator and does not jump.
In accordance with another embodiment of the present invention, the pair of brushes are positioned in a fan-like manner so that the pair of brushes open at an angle between 93xc2x0 and 115xc2x0 when the pair of brushes are mounted on the brush holder and before the commutator is disposed inside the pair of brushes. With this structure, when the commutator is disposed, the pair of brushes disposed in a fan-like manner are further opened and bent. Therefore, the brushes do not interfere with each other, and common brushes can be used as a pair.
In accordance with the present invention, the end sections of the pair of brushes may preferably be bent outwardly. For example, when the pair of brushes are mounted on the brush holder, the end sections of the pair of brushes outwardly bent at an angle of (180xe2x88x92D/2)xc2x0, where Dxc2x0 is an interior angle defined by the pair of brushes before the commutator is disposed inside the pair of brushes. With this structure, the end sections of the brushes can be extended, such that the end sections of the brushes become heavier. As a result, the brushes are maintained in contact with the peripheral surface of the commutator and do not jump from the commutator when the motor rotates at high speed.
Other features and advantages of the invention will be apparent from the following detailed description, taken in conjunction with the accompanying drawings that illustrate, by way of example, various features of embodiments of the invention.